A Microwave and Microfluidic Planar Resonator for Efficient and Accurate Complex Permittivity Characterization of Aqueous Solutions

A microwave resonator is presented as a microfabricated sensor dedicated to liquid characterization with perspectives for chemistry and biology. The nanolitter range aqueous solution under investigation is located on top of the planar resonator thanks to a microfluidic channel compatible with a future lab-on-a-chip integration. The interaction between the electric field and the liquid translates into a predictable relationship between electrical characteristics of the resonator (resonant frequency and associated insertion loss) and the complex permittivity of the fluid (real and imaginary parts). A prototype of the resonator has been fabricated and evaluated with de-ionized water/ethanol mixtures with ethanol volume fraction ranging from 0% to 20%. Good agreement has been reached between theoretical and measured electrical parameters of the resonator. The discrepancy on the resonant frequency is estimated to 0.5%, whereas the one on the associated transmission coefficient is lower than 1%. This translates into a maximum relative error on the real and imaginary part of the predicted relative permittivity of less than 6.5% and 4%, respectively, validating the principle of this accurate permittivity characterization methodology.